A single static image, or one of a sequence of images representing a moving scene, such as a video sequence, can lack detail, and appear “flat” or “soft”. Sometimes, this is a characteristic of the original image; for example, the scene lacks contrast or the lighting could have been very diffuse, resulting in an image with a low level of detail information. Significant loss of detail can also result from roll-off of high spatial frequencies caused by the finite aperture of the optical imaging system. In other cases, an image of adequate quality can be degraded by a transmission or coding system, resulting in loss of detail. Examples include transmission paths with high-frequency loss, or coding with composite television systems such as NTSC, PAL, or SECAM. Another example of coding that can reduce detail is compression, using a compression technique such as JPEG or MPEG.
Clearly, an image that lacks or has lost detail is inferior to one with an appropriate level of detail. However, more important than the actual loss of detail is the result that the image can appear “flat” or “lackluster.” A television viewer often will reject such an image in favor of a different program that contains images having greater detail.
Detail in an image can be enhanced by increasing the gain of the high spatial frequencies, but a simple system that increases detail by simply increasing the gain method will risk creating overshoots and ringing, that can introduce visible and objectionable artifacts. An image processed in such a manner will have a higher level of detail. However, such an image will likely be perceived as inferior by a viewer because of the presence of such artifacts.
Thus, there is a need for a technique to increase the detail and subjective appeal of an image without producing objectionable artifacts.